In recent years, in an electro-photographic image forming apparatus such as a copier and a printer, to uniformly charge a photosensitive drum or to transfer a toner image and separate sheets of paper, a charger by corona discharge is used. In the charger by corona discharge, when in use, a corona discharge member which is in a wire shape or in a needle shape installed on a metallic plate is contaminated.
When the corona discharge member of the charger is contaminated, uneven charging of the photosensitive drum occurs and the transfer efficiency of a toner image is reduced, thus an image fault is caused or a separation error of sheets of paper occurs. Therefore, conventionally, there is a charger having a cleaning mechanism for cleaning the corona discharge member.
As a cleaning mechanism, a mechanism for attaching a cleaning member to a wire member meshing with a pulley driven by a motor and rotating back and forth and moving back and forth while keeping the cleaning member in contact with the corona discharge member has been developed. The alternating movement of the cleaning member is controlled by controlling the motor by detecting arrival of the cleaning member at either of the ends of the moving range thereof.
In such a cleaning mechanism, during cleaning, it is important that the cleaning member surely moves back and forth overall the total length of the corona discharge member. Further, when the cleaning member stops, it is important that the cleaning member is surely located at either of the ends of the alternating movement.
When the direction of the alternating movement is inverted before the cleaning member reaches either of the ends of the corona discharge member, the whole area of the corona discharge member cannot be cleaned and an uncleaned area is left. Further, when the stop position of the cleaning member is shifted from the ends of the alternating movement, at time of charging, corona discharge by the corona discharge member is interrupted by the cleaning member and normal charging cannot be obtained.
On the other hand, as an image forming apparatus such as a copier or a printer, an image forming apparatus of a tandem type for forming toner images respectively on a plurality of photosensitive drums arranged in parallel, then multiple-transferring the toner images on one sheet of paper, thereby obtaining a color image is known. In such an image forming apparatus of a tandem type, an apparatus for moving back and forth respectively cleaning members of a plurality of corona discharge members arranged in parallel by a common motor is developed.
When driving a plurality of cleaning members by the common motor like this, it is necessary for the cleaning members to move back and forth in synchronization with each other. Therefore, the motor detects arrival of each cleaning member at either of the ends of the alternating movement and then is controlled so as to drive them inversely or stop. Namely, the motor is in the drive state before all the cleaning members reach the ends of the alternating movement.
However, in the aforementioned image forming apparatus of a tandem type, when an optional cleaning member reaches either of the ends of the alternating movement, the belt for supporting the cleaning member is stopped, so that by stop of the pulley meshed with the stopped belt, the motor is locked and an overcurrent flows through the motor and there is a fear that the motor is induced to fail.
Or, when the detection means of the cleaning member fails, although the cleaning member reaches either of the ends of the alternating movement, the motor is not controlled to stop. Therefore, before the error is detected and the motor is stopped, the lock state of the motor due to stop of the pulley is continued and the overcurrent continues to flow and there is a fear that the motor may fail due to the fault of the detection means.
Further, when the drive force of the motor is larger than the load of the belt and after the cleaning member reaches one end of the alternating movement, the pulley continues to rotate, a tooth skip of the belt occurs. Further, in such a case, a load is applied to the motor and there is a fear that the motor is induced to fail.
Therefore, in a charger having a cleaning mechanism, when the cleaning member reaches one end of the alternating movement of the corona discharge member and the belt is stopped, a charger, a process unit and an image forming apparatus for reducing the load of the motor during driving and preventing the motor from damage are desired.